1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with an improved dynamic secondary seal assembly for a reactor coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft.
The middle shaft seal section includes three tandemly-arranged mechanical seal stages--or lower, middle and upper sealing assemblies. The sealing assemblies are located in a seal housing concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically maintain a high positive pressure (2500 psi) boundary between the seal housing interior and the outside thereof during normal pump operating conditions while ensuring that the pump shaft can rotate freely within the seal housing. The lower sealing assembly which performs most of the pressure sealing (approximately 2250 psi) is of the non-contacting hydrostatic type, whereas the middle and upper sealing assemblies are of the contacting or rubbing mechanical type. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in MacCrum U.S. Pat. No. 3,522,948, Singleton U.S. Pat. No. 3,529,838, Villasor U.S. Pat. No. 3,632,117, Andrews et al U.S. Pat. No. 3,720,222 and Boes U.S. Pat. No. 4,275,891 and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The reactor coolant pump thus includes three mechanical seal stages that comprise a controlled leakage seal system. The reactor coolant pump also includes a plurality of secondary seals. Three of these secondary seals, located one per mechanical seal stage, are dynamic seals. The dynamic nature of these secondary seals is due to the fact that they are interposed between respective parts of the mechanical seal stages which undergo axial movement relative to one another. As they perform their sealing functions, the dynamic secondary seals must also accommodate several oscillatory movements induced in them by the mechanical motion and thermal expansion of the rotating pump shaft.
Although the three mechanical seal stages operate at quite different pressure conditions, the preference traditionally has been to provide the three dynamic secondary seals located at the three mechanical seal stages with substantially the same seal design. Originally, each secondary seal was a bare annular O-ring utilized between the relative axially moving parts of the respective mechanical seal stage. However, this first seal design did not function properly at all three mechanical seal stages due to its inability to accommodate the various dynamic conditions induced by the different pressures at the different seal stages. Subsequently, a second seal design was employed, being comprised of an outer annular O-ring and an inner annular liner composed of Teflon material and having a L-shaped cross-section. The liner provided a seat for the O-ring and slidably engaged the relative movable part of the respective mechanical seal stage. This second seal design was found to have improved performance but encountered difficulties in assembly. A third seal design, which has been used up to the present, was then substituted. This third seal design employed an outer annular O-ring also. But instead of the annular liner having the L-shaped cross-section of the second seal design, the third seal design used an annular Teflon liner having a double delta cross-sectional shape. Although the third seal design overcame the assembly problems of the second seal design, it still presented a sliding interface like the second design which tended to degrade and leak and to permit migration of dirt and foreign matter into the interfaces of the secondary seal.
Consequently, a need exists for an alternative construction of the dynamic secondary seals for the lower, middle and upper mechanical seal stages which will avoid the above-mentioned problems without introduction of a new set of problems in their place.